This invention relates to systems and methods of tuning a narrow-band (high Q) element or device over a relatively wide band of frequencies. More particularly, this invention relates to a series tuned power oscillator for driving a piezoelectric transducer element in which the resonant frequency of oscillation may be changed on every alteration of the sinusoidal output.
A power factor corrected piezoelectric acoustic transducer system is inherently narrow-band. The circuitry of the driving amplifier forms a tuned circuit with the transducer. The transducer and tuning elements provide a resonant frequency in the middle of the acoustic range of interest. At this resonant frequency, the output circuitry transfers the maximum power to the real part of the transducer load to be radiated as acoustic energy. At frequencies other than the resonant frequency, efficiency is compromised because more energy is stored in inductors and capacitors rather than being applied to the real part of the transducer load.
Thus, frequencies at or near resonance are the frequencies of greatest efficiency. Static tuning in known devices is acceptable when an application needs only a single frequency of transmission. Unfortunately, it is often necessary to transmit a wide frequency range of signals with a single transducer. In this case, the system is tuned to the center of the band of interest and the power amplifier output is varied to maintain relatively constant acoustic (real) output as the distance (in terms of frequency) from the resonant frequency increases. Since the power factor correction system has a narrow band, the system becomes extremely inefficient when operating at frequencies other than resonance. This inefficiency results in large power losses as the required bandwidth increases. Because of this problem, known systems have been designed with a large number of tuned transducers so that each transducer covers only a narrow range of frequencies while maintaining reasonable efficiency.
The present invention advantageously provides a relatively broad band of operating frequencies for a single transducer while maintaining reasonable efficiency.